The present invention relates to image sensors for detecting images with sub-wavelength resolution, and to methods of manufacturing the same.
As early as some decades ago, the Russian scientist, V. Veselago predicted the existence of materials with negative refractive indices n=c0/c=√∈rμr with c0 signifying the speed of light in vacuum, and c the speed of light in a material. Furthermore, ∈r signifies the electric conductivity or permittivity, and μr the magnetic conductivity or permeability of the material. At a negative refractive index n, the Poynting vector {right arrow over (S)} points in the opposite direction of the wave vector {right arrow over (k)}, and the wave vector, the electric field strength {right arrow over (E)} and the magnetic field strength {right arrow over (H)} form a left-handed tripod. Hence, materials with a negative refractive index are also referred to as so-called left-handed materials. The opposite directions of Poynting vector {right arrow over (S)} and wave vector {right arrow over (k)} result in energy transport against the wave and/or light propagation. At the transition from a left-handed material to a medium with a positive refractive index n, the light is not only refracted toward the perpendicular, but even beyond that.
Such so-called left-handed materials may be obtained when both, negative permittivity ∈r and negative permeability μr, are present in a material, so that the refractive index n becomes negative. With such a left-handed material, for example, an ideal lens or a so-called superlens can be constructed. It is characterized in that a point-like source has a point-shaped image, i.e. works completely without diffraction.
A classic optical system, such as shown in FIG. 12, is diffraction-limited.
FIG. 12 shows an objective 10. At a working distance or object-side distance f1 to the objective 10, there is a plane with an object to be imaged, i.e. an object plane 12. An image plane 14 is at an image-side distance or image distance f2 to the objective 10. If an object or the object plane 12 is in an object space having the refractive index n, a maximum resolution between two points having a distance d is obtained at
                    d        ≥                              0            ,                          61              ⁢                                                          ⁢              λ                                                          n              ·              sin                        ⁢                                                  ⁢            α                                              (        1        )            with the arrangement shown in FIG. 12, wherein λ signifies the wavelength of the light illuminating the object plane 12, and n sing the numerical aperture of the objective 10.
It can be seen from equation (1) that classically there are two ways of increasing the resolution of the optical system shown in FIG. 12. On the one hand, illumination with light of a shorter wavelength λ, can be chosen, such as with UV light or X-ray light in lithography, or so-called immersion liquids having a refractive index n increased with respect to air can be used in the object space, i.e. in the space to the left of the objective 10. However, even in the case of immersion liquids with a high refractive index n, it is smaller than two, and, hence, the generally achievable resolution lies in the range of about half a wavelength λ/2.
Optical systems with a resolution in the sub-wavelength region, i.e. with resolutions clearly smaller than λ/2, have already been demonstrated in the past. For example, the diffraction limitation described in the foregoing may be bypassed by methods of near-field optics. Common near-field microscopes have a resolution of less than λ/10.
At an observation distance to the objects that is much smaller than the wavelength λ of the light, the classic diffraction theory is no longer valid. Hence, the resolution here can no longer be limited due to diffraction either. This means that one practically bypasses the Rayleigh criterion in near-field microscopes and thus obtains resolutions below λ/2. The area of the light field lying close to an object to be examined is referred to as near field. So as to be able to acquire additional information from the near field, so-called evanescent field components have to be converted to propagating field components. The non-propagating component of the near field is generally referred to as evanescent field. The evanescent field drops exponentially to the surface normal of the radiating body. Thus, every illuminated object produces an evanescent field and a propagating one. For example, a purely evanescent field can be observed in the case of total reflection. If an incident light beam is reflected totally at an interface of an optically denser medium to an optically thinner medium, the field cannot become zero abruptly on the side of the optically thinner medium due to the continuity condition, but drops exponentially into the half-space of the optically thinner medium. In general, the evanescent field has already disappeared at a distance of about λ/2 from the interface of the two optical media. Yet it is exactly this field which contains information on structures below the classic resolution limit.
In order to convert evanescent field components to propagating field components, for example, a scattering center may be introduced into the near field. In this scattering center, dipole oscillations are excited by the evanescent field, so that again evanescent and propagating field components develop as a result of the interaction of the scattering center with the near field of the object. A further possibility is the scan of an object surface with an optical probe having a single-mode fiber, at the end of which there is an aperture having a hole diameter of about 40 nanometers. The light exiting this waveguide impinges on the object plane and thereby changes its evanescent field. A remote receiver and its signal processing register this change in the evanescent field, from which the refractive index n and transmission and reflection coefficients can be calculated. Various methods of measuring the near field are described, for example, in the dissertation “Eine hochauflösende optische Nahfeld-Sonde für Fluoreszenzmessungen an biologischen Proben” by Heinrich Gotthard Frey.
Only in the last few years have the theoretical properties of the left-handed materials, as described in the introductory section, been verified practically in experiments. Optical left-handed structures that image object structures in the sub-wavelength range, so-called superlenses have been realized. These optical structures with a negative refractive index transmit the evanescent field of an illuminated object, which carries information on spatial frequencies (e.g. of structures smaller than the wavelength λ of the light used), and image it from the object plane into the image plane in an almost lossless manner. These left-handed materials reconstruct the evanescent fields of sub-wavelength structures.
Presently, no materials with a negative refractive index or negative permeability μr are known to exist in nature. However, these properties can be achieved artificially with so-called metamaterials or photonic crystals, which have small periodic structures significantly smaller than the illumination wavelength λ, so that the electromagnetic waves only experience effective material properties. Some experiments confirming the theory of left-handed materials are described in the following publications. Hyesog Lee, Yi Xiong, Nicholas Fang, Werayut Srituravanich, Stephane Durant, Muralidhar Ambati, Cheng Sun and Xiang Zhan: “Realization of optical superlens imaging below the diffraction limit”, Wenshan Cai, Dentcho A. Genov, and Vladimir M. Shalaev: “Superlens based on metal-dielectric composites”, Gnnady Shvets: “Band engineering using electrostatic resonances applications to superlensing”.
Images appearing in an enlarged form with superlenses are also known, the publication “Magnifying Superlens in the visible frequency range”, e.g. shows theoretical and experimental results.
While the employment of left-handed materials for achieving resolutions in the sub-wavelength range is known, however, suitable image sensors to detect complete two-dimensional images of object samples in the sub-wavelength range are missing at present. In the previously described near-field microscopes, an object sample may only be scanned point-by-point, so that point-by-point two-dimensional scanning of the sample surface is needed in order to obtain a complete image. Moreover, transmitting the evanescent field onto a photoreceiver with the aid of additional optics, such as waveguides or lenses, is relatively problematic. The weak and, with increasing distance, exponentially decaying evanescent field is weakened further through the transmission onto the photoreceiver, which leads to low measurement accuracy. Furthermore, in conventional near-field detection, only static objects can be detected, since either exposing a film-like layer or scanning an object is needed.